A popular method of heating a room is to dispose an electric radiant heating element inside a floor of the room and then allow the heat generated in the floor to radiate in toward the room. In this manner, the floor of the room, especially a tile floor, does not remain cold while the temperature inside the room is comfortable, and thus a person walking on the floor with bare feet does not suffer an unpleasantly cold feeling in his or her feet.
Unfortunately, the standard process for installing an electric radiant heating element inside a tile floor is time consuming and requires care to avoid damaging the electric radiant heating element. The process begins by determining the size and length of the heating element to be used. Such electric radiant heating elements generate heat by resisting the flow of electricity through them. Because the electrical resistance of a material depends on the cross-sectional area and length of the material in the direction that the electricity flows, electric radiant heating elements are sized according to the specific application that they will be used for. For a specific heating requirement, one determines the length of the heating element needed. The length depends on the area of the floor or wall and the desired spacing of the element within the floor or wall. As the spacing between portions of a heating element increases, the temperature that one can obtain in the room decreases. Typically, one loses about 5 degrees for each 0.5 inch increase in spacing. Once the area of the floor or wall and the spacing between portions of the heating element have been determined, one selects the heating element whose gauge (diameter) in combination with its length provides the desired resistance (heat).
Because the resistance of the heating element depends on both the cross-sectional area and the length of the element, one cannot cut an element to reduce its length. To do so would decrease the resistance of the heating element and thus increase the current flowing through the heating element's electric circuit, which can cause serious damage. So, many heating elements are made to a specific, common gauge and length combination; not to a gauge and length combination that is specifically designed for a specific room. Because of this, positioning the heating element in the floor requires flexibility, which is provided by metal straps specifically mounted to a backer board as discussed below.
Installing an electric radiant heating element over a plywood or concrete sub-floor typically involves five activities that are time consuming and if not performed carefully can damage the electric radiant heating element.
First, one applies a heavy tile backer board that is awkward to handle. The backer board typically weighs about 30 lbs. and is three feet by five feet by ¼ of an inch thick. One applies the backer board to the plywood by spreading a thinset mortar (a cement based adhesive), laying the backer board on top of the mortar, and then fastening the tile backer board to the plywood with screws or nails. To complete the application of the backer board, one then covers the joints between each board with a self adhesive fiberglass mesh tape and then fills in the joints and covers the tape with thinset mortar.
Next, one installs furring strips at the perimeter of the room that one will use to know when enough cement underlayment has been applied over the electric radiant heating element to protect the electric radiant heating element from damage while the floor's construction is completed. A furring strip is typically ¼ of an inch high by 1.0 inch wide, and is installed by cutting the strip to a desired length and then fastening the strip to a backer board with one or more nails. Typically, a room requires four or more furring strips.
Next, one fastens to the backer board a metal strap that includes multiple tabs for holding the electric radiant heating element to the backer board. The metal straps are typically fastened to the backer board with a nail or screw at about every six inches along the strap's length. This part of the installation process can be very time consuming because one must first determine where on the backer board to fasten the metal strap to provide the desired electric radiant heating element spacing. This specific mounting of the metal straps to the backer board provides one with the flexibility needed by allowing one to mount the metal straps anywhere on the backer board. If the floor includes an odd-shaped area, or if the electric radiant heating element is to be operatively connected to another room, then before one can determine where on the backer board to fasten the metal strap, one must wait until the electric radiant heating element has been mounted to the remaining area of the room or the other room. Also, this part of the installation process can be very time consuming because one then has to fasten each metal strap to the backer board. For a 10 ft by 10 ft room one would have to attach at leats two 9.5 ft long straps to the backer board, each strap having at least 19 nails or screws driven through the strap and into the backerboard.
Next, one inserts the electric radiant heating element into the tabs of the metal straps to mount the electric radiant heating element to the backer board. This part of the installation process requires care to not damage the electric radiant heating element on sharp edges of the metal straps and their tabs. The sharp edges can cut the protective exterior of the electric radiant heating element and can pinch the element, both of which can prevent the element from operating. Furthermore, one must be careful not to trip on or drop anything on the element to also avoid damaging the element.
Finally, one covers the electric radiant heating element with a cement underlayment, using the furring strips previously fastened at the perimeter of the room to determine when no more underlayment is needed. As one spreads the underlayment throughout the floor, one walks and kneels on the cable. Because this can also potentially damage the electric radiant heating element, one must spread the underlayment with care. Also, because the furring strips are attached at the perimeter only, one must exercise skill when spreading the underlayment to ensure that the thickness of the underlayment is consistent throughout the floor's area.
The process of installing an electric radiant heating element over a concrete sub-floor is very similar to the process described above except one applies a cork or foam over the concrete slab to insulate the slab from the heat radiated by the electric radiant heating element, and then applies a thin coat of cement underlayment directly to the surface of the rubber or vinyl membrane to give the floor some texture for the thicker layer of cement underlayment to adhere to. In addition, fastening the metal strap to the concrete sub-floor requires concrete nails or concrete screws whose installation requires pre-drilled holes in the concrete.